Filaggrin is a major product of terminally differentiating mammalian epidermal cells that is thought to be involved in the aggregation and specific alignment of keratin intermediate filaments during the final stages of differentiation. Thus, filaggrin is an important example of an intermediate filament-associated protein. We have isolated both CDNA and genomic clones which show that filaggrin is expressed initially as a large polyprotein precursor, profilaggrin, which is subsequently proteolytically processed into individual functional filaggrin molecules. The structure of the gene for human profilaggrin has now been settled; it consists of 3 exons separated by 2 introns, the first of which is huge (about 10 kbp). The amino-terminal end of human and mouse profilaggrins possess two functional calcium-binding domains of the EF-hand type. There is evidence for multiple alternate splicing of introns and exons, giving rise to a series of other calcium-binding proteins of as yet unknown function. We have constructed genomic clones for the production of transgenic mice. We have begun a systemic analysis of regulatory sequences that control the expression of the profilaggrin gene system. We have determined that filaggrin aggregates keratin and vimentin intermediate filaments by ionic interactions.